Available tele- and videoconferencing systems have limited abilities to handle sound field signals, e.g., signals in a spatial sound field captured by an array of three or more microphones, artificially generated sound field signals, or signals converted into a sound field format, such as B-format, G-format, Ambisonics™ and the like. The use of sound field signals makes a richer representation of the participants in a conference available, including their spatial properties, such as direction of arrival and room reverb. The referenced applications disclose sound field coding techniques and coding formats which are advantageous for tele- and video-conferencing since any inter-frame dependencies can be ignored at decoding and since mixing can take place directly in the transform domain.
It would be desirable to provide an audio coding format allowing at least a simpler and a more advanced decoding mode (e.g., decoding into mono audio and decoding into some spatial format) while eliminating unnecessary processing and/or transmission of data when the simpler decoding mode is the relevant one. The referenced application by Cartwright et al. describes a layered coding format and a conferencing server with stripping abilities, e.g., a server adapted to handle packets susceptible to both relatively simpler decoding and more advanced decoding, by routing only a basic layer of each packet to conferencing endpoints with simpler audio rendering capabilities. It would be desirable for the stream of complete packets to fulfil a first bitrate constraint and for the stream of stripped packets (the basic layer and any header structures and the like) to fulfil a second bitrate constraint at all times. Finally, it would be desirable for the audio coding format to approach the coding efficiency of non-layered formats.
All the figures are schematic and generally only show parts which are necessary in order to elucidate the invention, whereas other parts may be omitted or merely suggested. Unless otherwise indicated, like reference numerals refer to like parts in different figures.